1. Field of the Invention
The present invention relates to an erbium-doped silica which is suitable for use in the manufacturing of optical fibers which are applied to optical fiber technologies such as the direct optical amplification and the like, and particularly relates to a manufacturing method for erbium-aluminum-phosphorus doped silica which utilizes aluminum, phosphorus, and the like, as co-dopants in addition to erbium, and which makes possible an increase in gain wavelength band.
2. Background Art
The direct optical amplification by means of optical fibers doped with rare earth metals is an example of an optical fiber technique which has attracted attention of late. FIG. 6 shows an example of such amplification technique; in the amplifier shown in FIG. 6, an pumping light 3 from an pumping light source (not shown in the drawing) is incidented into an optical fiber 1 (particularly hopeful as this optical fiber 1 are Er-doped single mode fibers objected for optical amplification in the 1.55 .mu.m wavelength band), using a WDM type optical coupler 2, and a signal light 4 is introduced simultaneously through an isolator 6a. By means of this, the energy of the erbium ions excited by pumping light 3 is applied to signal light 4 by means of induced emission, and the signal light amplified by means of this passes through an isolator 6b and is transmitted to fiber channel 5.
However, simple erbium-singly-doped optical fibers having a core doped solely with erbium ions have a gain wavelength band which is unstable in the vicinity of the 1.55 .mu.m wavelength band (1.53 .mu.m-1.56 .mu.m), as shown by line A in the graph of FIG. 7, when a small signal is used; because of this, such fibers are not suitable for use as materials for the manufacture of an optical fiber 1 constituting the above optical amplifier, which requires signal light having superior gain wavelength band stability.
An erbium-aluminum co-doped optical fiber having a core which has been subjected to the co-doping of erbium ions and aluminum ions, which stabilize the gain wavelength band of the erbium ions, has been proposed. This erbium-aluminum co-doped optical fiber is known to have a small wavelength dependency of the gain in the vicinity of the 1.55 .mu.m wavelength band, in comparison with the erbium-singly-doped optical fiber, as shown by line B in the graph of FIG. 7.
A VAD method, for example, is preferably employed in the manufacture of the erbium-aluminum co-doped optical fibers described above. In this method, first, as shown in FIG. 8, a glass seed rod 12 is placed within a reaction chamber 11. Next, oxygen and hydrogen gas are supplied to a burner 13 which is provided on this reaction chamber 11, and a flame is formed; this flame is directed to the lead end of the glass seed rod 12, SiCl.sub.4 gas, consisting of vaporized liquid form SiCl.sub.4, is supplied through the burner 13, this is subjected to pyrolysis, SiO.sub.2 microparticles (soot) are formed, and this soot is deposited on the lead end of glass seed rod 12 for forming a soot preform 14. Next, this obtained soot preform 14 is dipped in an alcohol solution of ErCl.sub.3 and AlCl.sub.3, this is desiccated, this is heated and made transparent in an atmosphere of He or the like, and a preform in which erbium ions and aluminum ions are co-doped is thereby produced.
Next, an optical fiber preform was manufactured using the above preform as a core preform. For example, methods were employed such as: a method in which the above core preform was inserted into a hole for core preform insertion provided in the central portion of a separately produced cladding preform, this was fused to form an integral structure, and an optical fiber preform was thus formed; a method in which soot for cladding was deposited on the outer circumference of the above core preform, thus obtained soot preform for cladding was heated so as to become transparent, and cladding thus formed, thus forming an optical fiber preform; and like methods.
Finally, this optical fiber preform was subjected to fiber drawing, and an erbium-aluminum co-doped optical fiber was manufactured.
However, in the above methods, when the doping concentration of AlCl.sub.3 in the silica exceeds 3 wt %, segregation of the AlCl.sub.3 occurs, and a crystal of the AlCl.sub.3 is precipitated within the silica, so that a problem existed in that AlCl.sub.3 could not be doped at high concentrations.
A method is known for the prevention of this AlCl.sub.3 segregation phenomenon in which a phosphorus compound (for example, POCl.sub.3) having the effect of suppressing the segregation of AlCl.sub.3 was emitted through the burner 13 together with SiCl.sub.4 and thus applied in soot preform 14, and after this, this was impregnated with in an alcohol solution of ErCl.sub.3 and AlCl.sub.3 by means of a VAD dipping method identical to that described above, this was then desiccated, and soot preform 14 was heated and made transparent.
In the above method, when soot was deposited on the lead end of glass seed rod 12, a phosphorus compound having the effect of suppressing the segregation of AlCl.sub.3 was doped within soot, and thus formed soot preform 14, having a phosphorus compound doped therein, was heated and made transparent in a mixed atmosphere of gasses produced by the sublimation of ErCl.sub.3 and AlCl.sub.3, thus forming a preform; an attempt was thus made to realize a high doping concentration of aluminum ions in the silica preform by means of preventing the crystallization of AlCl.sub.3.
However, as explained above, when a phosphorus compound is doped in soot at the time of the deposition of soot, the viscosity of the each soot having a phosphorus compound doped therein is reduced, and as a result of this reduction in viscosity of soot, the bulk density of the soot preform 14 which is formed increases in an undesirable manner, and as a result of this, there was a problem in that the doping concentration of the aluminum compound was reduced.
Furthermore, the operation of doping a phosphorus compound within soot while continuously depositing soot on the lead end of glass seed rod 12 is conducted using burner 13, so that there was a problem in that, as a result of the irregularities of the heating temperature of this burner 13, the doping concentration of the phosphorus compound, and the concentration of the other dopants doped in the liquid phase later, was not uniform between the central region of the soot preform 14, which had a high heating temperature, and the peripheral area of the soot preform 14, which had a low heating temperature.
Furthermore, differences in viscosity resulting from differences in phosphorus concentration were large, so that it was difficult to heat the soot preform and make it transparent at the time of sintering, and a problem existed in that it was difficult to obtain solid glass, as the shrinkage of the inner side of the soot preform progressed even though the outer side thereof remained in a soot state.